Calcium phosphate, Ca10(PO4)2(OH)2, also known as hydroxyapatite (HA), is the major component of natural bone tissues and is characterized by an excellent bio-compatibility; as a result, it is currently widely used in various forms and shapes in bone and tissue engineering [1]. Nonetheless, the mechanical properties of hydroxyapatite, such as its low toughness of about 0.8-1.2 MP and low flexural strength (less than about 140 MPa), limit its use in the regeneration of various parts of the bone systems, especially those under significant mechanical tension. Moreover, the difficulty of generating three-dimensional (3D) bone re-growth structures from HA further restricts its use in the regeneration of complex bone systems [2].
In order to overcome these limitations, a number of HA-based composite materials has been developed and studied based on both natural and synthetic polymers [3-5] with inorganic compounds such as alumina (Al2O3) [6, 7], titanium (Ti) or Ti alloys [8, 9]. The characteristics of the carbon nanotubes (CNTs), such as high aspect ratio (hundreds or thousands) [10, 11], a good electrical [12] and thermal conductivity, and excellent mechanical properties (modulus of elasticity of about 1.25 TPa) [13], suggest that these materials could be an excellent candidate for a scaffold or as a doping agent in the composites used for bone engineering. It has been shown [14] that the utilization of CNTs along with HA and polymethyl methacrylate (PMMA) resulted in the development of a new composite material with superior mechanical properties as compared to those without the CNT addition for biomedical scaffolding in bone engineering and regeneration. Previous studies have reported the use of both single-walled carbon nanotubes (SWCNTs) [15] and multi-walled carbon nanotubes (MWCNTs) for such applications [16, 17].
However, it remains extremely difficult to accomplish a perfectly homogeneous composition of the CNT-based composites by traditional mixing technology, given the possible development of defects in the nanotube walls, with major influences on the macroscopic properties of the composites. To overcome this limitation, the development of such composites is performed through sintering [18] or in-situ growth of the CNTs in the HA matrix [19] or over the catalytic Fe, Ni, and Co nanoparticles supported on the surface of HA [20].
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.